1. Field of the Invention
This invention relates to a continuous process for the production of polymers having small linear expansion coefficients and high tensile moduli (Young's moduli), and more particularly to a continuous process for the production of a polyoxymethylene (POM) rod or tube having a high tensile modulus.
2. Prior Art
Popular polymeric materials, such as polyethylene, polypropylene, polyoxymethylene, polyesters and nylon, have been frequently processed through hot drawing to obtain ultrahigh-modulus products. A variety of hot drawing processes has hitherfore been known in the art, including a process wherein a material is allowed to contact with a hot pin or hot-shoe, a process wherein a material is passed through a hot tube, a process wherein a material is allowed to contact with a rotating hot roller, and a process wherein a hot pin and a hot-shoe are used in combination. However, these known processes have a common disadvantage that these processes can be applied only to process a material in the form of a fiber having a diameter of less than about 0.1 mm or in the form of a film having a thickness of less than about 0.1 mm, since the material is heated from the outside in any of these known processes leading to the result that the inside of the material cannot be heated sufficiently uniformly when the material is processed in the form of a thicker fiber or film. Moreover, the tensile modulus (Young's modulus) of a product polyester fiber processed through any of these known processes is in the order of 20 GPa (Giga Pascals) at the highest.
U.S. Pat. No. 3,364,294 discloses a process for drawing a heavy denier polyester tow (a bundle of filaments) while heating the filaments from the inside thereof by dielectric heating to obtain a product having a tensile modulus of 14 GPa. However, a process for the production of a rod or tube made of an ultrahigh-modulus polymer by drawing the rod or tube under dielectric heating has never been proposed by the prior art references including the preceding United States Patent referred to above.
Recently developed are processes for the direct production of rods or tube having ultrahigh-moduli from any of popular polymer materials. One of the processes recently developed is a so-called hydrostatic extrusion process in which a solid polymer material is forcibly extruded through a tubular die having an inlet of conical shape by the application of hydrostatic pressure. The other of the recently developed processes is a die drawing process in which a solid polymer material is pulled out through a narrow die to be drawn. For example, according to the hydrostatic extrusion process, a POM (polyoxymethylene) rod having a tensile modulus of 24 GPa was produced at a production speed of 0.01 m/min. (Reference should be made to P. D. Coates and I. M. Ward, J. Polym. Sci. Polym. Phys. Ed., 16, 2031 (1978).) On the other hand, according to the die drawing process, a POM rod having a tensile modulus of 23 GPa was produced at a production speed of 0.1 m/min. (Reference should be made to P. S. Hope, A. Richardson and I. M. Ward, J. Appl. Polym. Sci., 26, 2879 (1981).) However, the tensile modulus of a POM rod obtainable by these known processes is lower than 25 GPa. A further and fatal disadvantage common to these known processes is a low production speed which provides an obstacle for the processes to be practically utilized in a commercial scale.